Question

A hollow metallic sphere of radius 20 cm surrounds a
concentricmetallic sphere of radius 5 cm. The space between the two
spheresis filled with a non-metallic material. The inner and outer
spheresare maintained at 50oC and 10oC
respectivelyand it is found that 100 J of heat passes from the
inner sphere tothe outer sphere per second. Find the thermal
conductivity of thematerial between the spheres.